Ionization type smoke alarms and photoelectric type smoke alarms are commonly used in residential buildings. Each type has its advantages. Ionization type smoke alarms generally respond faster to flaming fires, while photoelectric (optical) type smoke alarms generally respond faster to smouldering fires. Although both ionization and photoelectrical smoke alarms meet the standards established by the fire protection industry, for improved protection authorities such as the National Fire Protection Association (NFPA) recommend that both types be used in the home. (NFPA “What you should know about Smoke Alarms” http://www.nfpa.org/assets/files//PDF/Public%20Education/NFPASmokeAlarmFactSheet.pdf) However ionization type smoke alarms tend to generate nuisance alarms when installed near kitchens. They are often activated to generate loud audible alarms or sounds during routine cooking procedures. Such nuisance alarms or sounds are very discomforting to the occupants. Nuisance alarms are the main reason occupants disable smoke alarms. A 2007 Seattle study found 20% of ionization alarms were non-functional one year after installation. (Mueller B. A. Sidman E. A. “Randomized Controlled Trial of Ionization and Photoelectric Smoke Alarm Functionality” Injury Prevention 2008; 14:80-86) Because disabled smoke alarms pose a major safety risk, there is a need in today's market for an ionization smoke alarm that is less likely to generate nuisance alarms. In contrast photoelectric smoke alarms are less likely to nuisance alarm. The same study found that only 5% of photoelectric smoke alarms were non-functional after the same period.
Currently occupants are advised to relocate an ionization smoke alarm away from the kitchen surrounds in order to minimise this problem. However relocation might not be possible in a small dwelling as the most important location for a smoke alarm, just outside the bedroom, might be close to the kitchen. When relocation is not possible, the occupant is advised to install a photoelectric type smoke alarm instead. Using photoelectric type smoke alarms reduces nuisance alarms but also reduces protection. For improved protection both types of smoke alarm should be used. Alternatively, the occupant is advised to install an ionization smoke alarm that features a “Hush” button. Hush buttons can deactivate/desensitize the smoke alarm for a short period. Unfortunately this, too, does not solve the problem since such buttons are beyond reach for most occupants due to positioning of the alarms on walls and ceilings. The smoke alarms used in the Seattle study all featured hush buttons. Even those who can reach the hush button are still at risk of becoming desensitized to the smoke alarm if it sounds frequently.
The applicant is aware of several proposals for overcoming above mentioned prior art problems. For example, the disclosures in patent references RU2207630 (Savushkin, V. A.), JP2006-202080 (Takashima, Hiromasa), JP2007-148694 (Sekine, Takehiro), BE1016841 (Tanghe, Freddy), GB2457696 (Bone D. G.), JP2010-198406 (Shinozaki, Ritsu) teach either a passive infrared motion Detector (PID) or a Doppler Effect motion Detector that automatically desensitizes a fire alarm during human presence in the area. Since most nuisance alarms occur during meal preparation and hence during human presence, these proposals alleviate the problem to some extent. However these proposals cannot be allowed to completely deactivate or significantly desensitize the alarm for an extended period of time. Doing so would create an unacceptable risk for the occupant and would not meet fire safety standards. This is because such motion Detectors are at risk of responding to pets or children or fire or other interference sources. Unfortunately the lower sensitivity limit for ionization smoke alarms, allowed by most authorities, is not low enough to block many nuisance alarms that commonly occur near the kitchen. (e.g. see Australian Standard 3786-1993, minimum sensitivity for ionization sensors=0.5 MIC×value) Thus, these proposals do not adequately solve the problem. Furthermore, because of the technology employed, all these proposals require at least two separate packages for implementation and are not suitable for drawing their power from the smoke alarm's own battery. This reduces their aesthetics and makes them expensive and hard to install. Also, the PID detectors described in the above patents are likely to see and perhaps respond to a fire or nearby interference sources due to their wide field of view. This could cause alarm desensitization for the wrong reason.
US 2010-0238036 (Holcombe, Wayne T.) discloses a fixed distance proximity detector inclusive in a standard smoke alarm. Unlike PID detectors, such a detector is relatively immune to interference sources since its detection zone is only a short distance below the smoke alarm. It could possibly be used to completely deactivate the alarm whilst still maintaining safety standards. However, for this very reason, it would not normally block nuisance alarms before they occur. Blocking would require a deliberate action by the occupant, such as a hand wave above the head and under the smoke alarm, before cooking commenced. Also, for some occupants, the proximity detection zone would be beyond reach.
U.S. Pat. No. 7,642,924 (Andres, John,) discloses a combination ionization sensor and carbon monoxide (CO) sensor functioning as a smoke alarm. The sensitivity of the ionization sensor changes according to the presence of CO. Since cooking tends to produce less CO than a real fire this technique can reduce nuisance alarms. However to screen against certain cooking activities, such as toasting bread or frying bacon, the CO threshold needs to be set quite high. Although this threshold is acceptable to fire safety authorities, it will nevertheless result in a significant loss in smoke alarm sensitivity which unnecessarily continues around the clock. Alternatively, if the smoke alarm sensitivity is maintained, it will suffer from a significant nuisance alarm problem near the kitchen.
Other multi-sensor fire alarms now arriving on the domestic market introduce heat, carbon monoxide (CO), rate of change measurements and other information, together with smoke sensor measurements, into an onboard algorithm for processing. These devices offer improvements but must still compromise on performance to mitigate nuisance alarms near the kitchen.
An additional problem manifests itself during low battery conditions of ionization and photoelectric smoke alarms as well as other types of alarms. When the battery in these alarms reaches a low power condition a smoke alarm will beep intermittently at about once a minute. This is to alert the occupant of the need to replace the battery. This often occurs in the early hours of the morning when low temperatures maximise the condition. This beep is loud enough to prevent or disturb sleep. As a result, the occupant often cannot postpone the battery change. Additionally the beep is very short in order to preserve the life of the already depleted battery. Because most dwellings are fitted with multiple smoke alarms the faulty smoke alarm can be very hard to find. Thus there is a need for an improved method of locating a smoke, or other, alarm in this condition.
US2010-0238036 (Holcombe, Wayne T.) discloses a method of providing the occupant with a feedback tone when the proximity detector is activated and the smoke alarm is in the low battery state. This system can help some occupants locate an alarm in such a state. However, as mentioned earlier, the proximity detector will be out of reach for other occupants. Thus this method will not always solve the low battery alert problem.